Solid propellant rocket motors are utilized for propelling various missiles and aerospace vehicles including the space shuttle and Titan launch vehicle. These rocket motors include a motor case which surrounds a solid propellant having a star or other shaped hollow core which defines a combustion chamber, an igniter assembly, which is usually mounted on top of the propellant, and a bottom mounted nozzle assembly. The igniter assembly initiates burning of the propellant generating hot combustion gases which travel through the core, initiating combustion along the exposed propellant surfaces. The propellant then burns radially towards the motor case, generating additional hot gases which exit through the nozzle assembly, thereby providing forward thrust.
The motor case that contains the propellant is generally made of a high strength material, such as steel or fiber reinforced composite. An insulator, which usually comprises one or more layers of a rubber material bonded to the case wall, prevents excessive heating and possible burn-through of the casing during operation. Generally, a liquified formulation is added to the insulator surface and partially cured, forming a thin liner which enhances bonding of the propellant to the insulator. The liquified propellant mixture is then prepared and poured into the motor case, contacting the semi-cured liner, with the propellant and liner then processed to effect a final cure, thereby firmly bonding the propellant to the liner and the liner to the insulated motor case. It is essential that the propellant be properly bonded to the case to prevent movement of the propellant during burning. Such movement could result in loose propellant clogging the discharge nozzle, resulting in failure of the engine. It is also imperative that no disbonds or voids occur in the propellant liner interface. Such imperfections may result in the propagation of cracks in the propellant as the motor is stressed during operation, which could provide additional burning surfaces, resulting in an unregulated increase in operating chamber pressure and possible engine failure.
Because of the potential catastrophic results, extensive non-destructive testing is done on solid propellant rocket motors to determine if any such imperfections exist. X-ray analysis of selected surfaces of the rocket motor are generally the preferred method of testing. Generally, an X-ray beam is transmitted through the rocket motor where it is variably absorbed, depending on the thickness, density and composition of the insulator/liner/propellant. Since the thickness is indeterminate after bonding and the chemical compositions are essentially equivalent, density differences provide the basis for analysis.
In particular, computed tomography is an especially preferred method of inspection which involves preparing numerous thin cross-sectional X-ray slices of the engine which are evaluated by a computer. Such an inspection method eliminates the superimposition of features that may tend to distort images and obscure critical faults. However, even utilizing computed tomography, it is difficult to observe the propellant to liner interface to assure good adhesion and bonding due to the choice of insulator material. Previously, an asbestos/nitride butyl rubber insulation material was used having a specific gravity of approximately 1.27, with most propellant mixtures having a specific gravity of approximately 1.1. In an X-ray analysis, the propellant liner interface was easily discernable. However, current insulators, which do not use asbestos, have a specific gravity of approximately 1.1. Since there is no significant density difference between the propellant, liner and insulator, the propellant liner interface cannot be easily detected during testing.